OBJECTIVE. The purpose of this study was to determine whether ultralow-radiation-dose chest CT can be used for quantification of lung density and for emphysema detection in participants undergoing lung cancer screening. SUBJECTS AND METHODS. Fifty-two patients were prospectively enrolled and underwent scanning twice with low-dose CT (reference parameters, 120 kV, 50 effective mAs) and ultralow-dose CT (reference parameters, 80 kV, 4-5 effective mAs). Images were reconstructed by filtered back projection (FBP) for low-dose CT and FBP and iterative reconstruction (IR) for ultralow-dose CT. Radiation dose was recorded. Image noise, mean lung attenuation, 15th percentile of lung attenuation, and emphysema index were measured in each image series and compared. Test characteristics of ultralow-dose CT in detecting more than subtle emphysema (emphysema index >= 3%) were calculated. RESULTS. The effective dose of low-dose CT was 2.1 +/- 0.5 mSv, and that of ultralow-dose CT was 0.13 +/- 0.04 mSv. Compared with the findings for low-dose CT, absolute overestimation of emphysema index was 7% on ultralow-dose CT images reconstructed with FBP and 2% on those processed with IR. The 15th percentile of lung attenuation was underestimated by 21.3 HU on ultralow-dose FBP images and by 5.8 HU on IR images. No relevant bias was observed for mean lung attenuation. Four patients (8%) had more than subtle emphysema. The emphysema index measured at ultralow-dose CT with FBP and IR had 100% and 100% sensitivity and 92% and 96% specificity in identifying patients with more than subtle emphysema at a cutoff of greater than 12.1% for FBP and greater than 6.7% for IR. CONCLUSION. Ultralow-dose chest CT performed for lung cancer screening can be used for quantification of lung density and for emphysema detection. IR improves the accuracy of ultralow-dose CT in this setting.